Handheld metal detectors typically have a search loop that is swept over ground to look for buried targets. Often the ground includes mineralization which creates a response in the detector. Some metal detectors include electronic filters which can distinguish between slowly varying mineralization (ground) signals and fast-response target signals. Optimum filter design depends on how fast the user sweeps the search coil, as this determines the speed of the target response. In the past, some metal detectors have had filters designed for a slow optimum sweep speed, while other detectors required a fast sweep speed.
Modern metal detectors also utilize analog and digital signal processing which further separate desirable target responses from undesirable ground responses using techniques commonly called ground balance and ground tracking. Additional signal processing is used to distinguish different types of targets, a technique known as discrimination. Both ground tracking and discrimination are susceptible to sweep speed. That is, both can be optimized if the sweep speed of the search loop is known. Other methods of signal processing can be triggered if certain loop motion occurs. For example, manual ground balance can be affected by “bobbing” the loop up and down, and target averaging can be applied when short, quick sweeps are used over the target.
Many detectors include either visual or tonal identification, or both. Visual identification can include a number of methods of displaying target response, some of which depend on the sweep speed of the loop, or knowing the direction of the loop sweep, or knowing the endpoints of the search sweep, that is, the points where the loop stops and reverses direction. Tonal responses can also be altered under different sweep situations, resulting in more varied or more accurate audio information.
In current metal detector designs, there are often provisions for changing filter parameters, search modes, ground tracking responses, signal processing methods, and visual display techniques, but these usually require manual intervention by the user and sometimes advanced knowledge of how the technology works, leading to user errors in setting up the detector.
Needs exist for improved metal detectors.